Fix spec problem with nil class name under rbx-19mode
Fix code that breaks under rbx in 1.9 mode

* For some reason specifying the arguments precisely allows
  for the correct arguments to be passed through. For some reason
  the original code causes an arity error, because only the key is
  passed through.

  I assume this is due to a bug inside the rbx parser, but I was not
  able to narrow it down further. To reproduce this error, use the
  parent commit and run the following:

    GIT_BRANCH=release-1.2 ruby -X19 -S bundle exec spec spec/public//associations/many_to_many_spec.rb

  The most interesting thing is that this error doesn't occur the
  first time the method is accessed. If you do a full spec, run the
  method is clearly called multiple times, and then something
  happens inside rbx that changes the arguments yielded into the
  block.